Superconducting Magnets theory and design. Guillaume Donnier-Valentin

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1 Superconducting Magnets theory and design Guillaume Donnier-Valentin Cryocourse September

2 Superconducting wire Critical parameters Critical temperature Critical magnetic field Critical current density Critical surface for superconducting materials Cryocourse September

3 Different type of superconducting wire LTC superconducting wire NbTI Nb 3 Sn, Nb 3 Al MTC superconducting wire MgB 2 HTC superconducting wire Bismuth tape YBCO tape Cryocourse September

LTC superconducting wire NbTi conductors (many fine filaments of a niobium-titanium (NbTi) alloy embedded in a copper matrix) The most commonly used (developed since the sixties) High performance Tc

4 LTC superconducting wire NbTi conductors (many fine filaments of a niobium-titanium (NbTi) alloy embedded in a copper matrix) The most commonly used (developed since the sixties) High performance Tc (0T) = 9,5K ; Tc (11T) = 4,1K Feasible industrially (Supercon, ASC..) 2000 t/year Easy to provide Competitive cost (around 300 $/km depend on wire diameter or 100 $/kg or 1 /ka/m) Guaranteed critical characteristic Easy to handle and no special precautions for use (like copper wire) Lot of choice (size, resistive matrix, copper proportion, multi or mono filamentary ) Isotropic properties Ø = 0.5 mm I c = 500 A J e = 2500 A/mm 2 (4.2 K ; 0 T ) Operate at 4 K or lower temperature Helium cryogenic High refrigeration cost 1W/700W Cryocourse September

5 LTC superconducting wire pe No. of Filaments Cu to SC Ratio Diameter (mm) Critical Currents 4.2K) at fields (Tesla,T) Bare Insulated 3T 5T 7T 9T Filament Diameter (µm) For our NbTi Coil 56S :1 54S We used 54S43 multifilament wires from supercon company 54S S43 NbTi Wire Good stability (1/3 of copper) and good performances Ic (A) K Æ 0.7 mm Æ 0.6 mm Æ 0.5 mm Æ 0.85 mm Coil design limited under 9.5 T at 4.2 K H(T) Cryocourse September

6 LTC superconducting wire In superfluid helium at 1.8 K we can reach up to 12 T BUT When operated at reduced temperatures and higher fields, the energy in the magnet can be increased by 50% or more. Consequently, the magnet might be irreparably damaged if a quench occurs and if the magnet is not sufficiently protected Cryocourse September

7 LTC superconducting wire Multifilamentary, Niobium-Tin (Nb3Sn) conductors Characteristics Are used when the field on the conductor is in excess of about 9 Tesla (90 kilogauss) Better critical characteristics than NbTi (Tc (0T) = 18,3 K; Tc (11T) = 10,4K ; Jc=10 6 A/cm 2 ) Allow to reach higher field (up to 18 T) Used for fusion application (ITER) (fields of 13 T are needed) Cryocourse September

8 Multifilamentary, Niobium-Tin (Nb3Sn) conductors Nb 3 Al LTC superconducting wire Difficult to wind The coil must be realized before the heat treatment of the wire (lead to impregnation problem) Very brittle after the heat treatment expensive $2000/kg ; 10 /ka/m ; only 20 t/year produced Cost more than NbTi magnets. Like for NbTi magnets, some improvement in performance can also be achieved by reducing the temperature of Nb3Sn magnets, but the increase in field is not as significant as it is in NbTi magnets Better mechanical properties in comparison with Nb 3 Sn Industrial development much less advance than for Nb 3 Sn Cryocourse September

Fe+Cu matrix Marketable, development in progress: Cost 100 /ka/m Different type

9 MTC superconducting wire MgB 2 Tc (0T) = 39K J E =3*10 4 A/cm 2 Fabrication process : Powder in tube (PIT) Long length available (several km) in Nb+Cu or Fe+Cu matrix Marketable, development in progress: Cost 100 /ka/m Different type of MgB2 wire Diameter 1-2 mm (Columbus Superconductors SpA) Cryocourse September

10 HTC superconducting wire Critical temperature around 100K Liquid nitrogen 77K or cryocooler Bismuth PIT tape (powder in tube) fil Bismuth PIT fil section Silver Bismuth Cryocourse September

HTC superconducting wire Bismuth PIT wire High anisotropy Available in kilometric length Tc = 110K (Bi 2223 ); Jc=5.

11 HTC superconducting wire Bismuth PIT wire High anisotropy Available in kilometric length Tc = 110K (Bi 2223 ); Jc= A/cm 2 (77K); Very brittle wire Minimum bend radius Problem with successive thermal cycling (ice in porosity which damage the wire during the following cooling) Expensive (due to silver matrix 5 /m) Bi /ka/m Cryocourse September

12 HTC superconducting wire Bismuth PIT wire Difficult to make solenoid coil due to rectangular cross section (critical current decreases rapidly when we applied a bend along the radial direction) Pancake coil Low mechanical properties (alloys silver Mg for matrix or used of stainless steel tape in parallel to reinforced the structure) Cryogenic cost less important than for superconducting LTC coil Cryocourse September

always a critical current at high field.

13 HTC superconducting wire Bismuth PIT wire High anisotropy B B In comparison with LTC superconductors this tape have always a critical current at high field. It can be used to do insert inside high field coil Cryocourse September

14 HTC superconducting wire Bismuth PIT wire Critical current is very sensitive to the magnetic field at high temperature, at low temperature it is much better Engineering critical current J e //c (A/mm 2 ) tape Bi 2223 PIT ASC 20 K 50 K 1 77 K 0, Cryocourse September B (T)

µm Nickel substrate textured by rolling Buffer layers are

15 HTC superconducting wire Coated conductors YBaCuO YBaCuO tape ; I c = 200 A YBaCuO 50 µm 10 mm Substrat Nickel 1 µm Nickel substrate textured by rolling Buffer layers are deposited then YBCO is deposited Cryocourse September

HTC superconducting wire Coated conductors YBaCuO High anisotropy High Tc and critical current density Tc=92K; Jc=5.

2K) Difficult to do (epitaxial growth of YBCO layers) For the moment, no real industrial process to do it in long

16 HTC superconducting wire Coated conductors YBaCuO High anisotropy High Tc and critical current density Tc=92K; Jc= A/cm 2 (77K) ; Jc= A/cm 2 (4.2K) Difficult to do (epitaxial growth of YBCO layers) For the moment, no real industrial process to do it in long length (100m) with constant critical properties MOCVD, PLD, No economical blocking (no silver like for PIT tape) Interesting but expensive for the moment > 200 /ka/m Cryocourse September

17 HTC superconducting wire Coated conductors YBaCuO I/Ic (0T) 1 0,9 0,8 0,7 0,6 0,5 YBaCuO ; perpendicular field (source Wu XD and al Los Alamos) Bismuth ; perpendicular field 0,4 0, B (mt) 77 K Cryocourse September

18 Comparison LTC and HTC Cost today: NbTi : 1 /ka/m ; Nb3Sn : 10 /ka/m MgB2 : 100 /ka/m Bisco : 200 /ka/m YBCO : 300 /ka/m Cryocourse September

19 Interest to do a superconducting magnet Compared to copper coil Generate intense magnetic fields with low electrical power input Current densities are high : superconducting magnet systems are quite compact, more light and occupy only a small amount of laboratory space very high field stability (superconducting switch) No losses in DC application, low electrical power consumption Cryocourse September

20 Superconducting magnet Small superconducting magnets are frequently used : to reach field intensities, stabilities or field shape that are not reachable with alternative magnets cost less than conventional magnets offering comparable or inferior performance. In large magnets cost in favor of superconducting magnets (based on the relative costs of power for operating the magnets). The cost becomes more favorable for superconducting magnets as the period of operation increases. For magnetic field intensities of 1 Tesla or less, without demanding stability requirements : Frequently better generated with water cooled copper coils with or without iron. Cryocourse September

21 H H max Superconducting Magnet Geometry for high field coil >20 T 0 NbTi Solenoid coil Nb 3 Sn Bismuth Pancake coil High field magnets could be wound with a combination of NbTi wires in the low field region followed with Nb3Sn wound in medium field region and using Bi2212 windings in the high field region. Cryocourse September

22 Laboratory superconducting User's specifications magnet design Magnetic field configuration (maximum field, field shape, compensated zone, stray field) Environmental constraints (geometrical, hole field size, weight, material used for mandrel, flange) Operating mode (DC or AC or Pulsed, long term stability superconducting switch) Cryocourse September

23 Laboratory superconducting magnet design First design approach Determine the winding configuration Ampere-turns needed to produce the field The stored energy The peak field in the conductor region An estimate of the magnetic forces and the needed mechanical structure The dependence of these parameters on the average current density in the winding Cryocourse September

24 Laboratory superconducting magnet design First design approach Cooling mode Pool boiling very efficient, direct contact with helium, we must used superfluid helium at 1.8 K but we need a solid and lick-tight helium vessel Forced flow internal cooling very efficient, cooling directly inside the conductor no helium vessel but difficult with internal joint Indirect cooling heat conduction through a compact winding from a restricted cooling loop or cryocooler simplest scheme but sensibly lower efficiency use for magnet operating in steady state regime Cryocourse September

25 Laboratory superconducting magnet design First design approach Stability Ability of a superconducting magnet to operate without accidental transition to the normal state under thermal disturbance Stability margin Cooling method Conductor characteristics Cryocourse September

26 Laboratory superconducting First design approach Stability magnet design Example : impregnated superconducting solenoid ; He pool boiling Maximum field on conductor : 9T Nominal current : 90 A Cryocourse September

27 Laboratory superconducting magnet design First design approach Quench protection Resistance or diode in parallel on the output of the coil Quench detector for certain coil with high energy Shut down the power supply in case of quench Cryocourse September

28 Electrical insulation In steady state there is no voltage across the coil But during the quench fast dump of current high voltage can be created and can damage the magnet Special requirement for cryogenic insulation Related to electrical, thermal and mechanical properties We mainly used Fiber glass, glass-epoxy composite (G10 G11), Kapton (polyimide film), araldite for impregnation, Mylar (polyester films, "polyethylene terephthalate"), teflon ("Polytetrafluoroethylene (PTFE) "), nylon Cryocourse September

29 Electrical insulation For Nb3Sn magnet, there is another difficulty : Thermal cycle at high temperature ( C) required to synthesize the superconducting phase Polymer insulation cannot be used before the thermal cycle Usually fiberglass spacers are used during thermal cycle this step is followed by a impregnation under vacuum Nb 3 Sn superconducting phase and fiberglass are very brittle after the heat treatment impregnation process needs special care Cryocourse September

30 Electrical insulation Table : Electrical and thermal properties for insulating materials (p 347 handbooks of cryogenic engineering) Cryocourse September

31 Impregnated NbTi superconducting solenoid Superconducting solenoid construction Theoretical design Mechanical part design, machining Winding and impregnation Protection Superconducting switch Magneto forming contact Cryogenic tests, training, field card 8 T coil with 0 field zone Cryocourse September

32 Theoretical design First approach Choose the most appropriated wire to do the coil (wire diameter, critical current) Maximum current define by the user (power supply, experimental set up) Maximum field and size of the hole field Estimation of number of turns to reach the user's specification Max field, volume, compensated zone, field shape, field homogeneity at the maximum, first estimation Calculation of magnetic induction in the center of a solenoid with constant field density N : nb of turns, I current NI B 0, H solenoid length H B h. J..ln 1 Impregnated NbTi superconducting solenoid Gauss cm Cryocourse September A h = ½ length (cm) J = current density in the superconducting wire (A/cm 2 ) = external radius / inner radius = half length / inner radius = packing factor

33 Estimation of energy Impregnated NbTi superconducting solenoid With L(µH)=2.2*N^2*D^2/(2.2*H+D) Solenoid length H(m) diameter D(m), avec N nb of turns For a solenoid having H/D>.2, with a degree of accuracy of 3%. Cryocourse September

more accurate work : Impregnated NbTi superconducting solenoid Thanks to a finite element software Flux2D, 3D (computation electromagnetic software) or FORTRAN programs We can calculate precisely the

34 more accurate work : Impregnated NbTi superconducting solenoid Thanks to a finite element software Flux2D, 3D (computation electromagnetic software) or FORTRAN programs We can calculate precisely the magnetic field in all place of the space (axial, radial and total magnetic field) Calculate inductance, energy Calculate Lorentz force distribution in the winding for stress analysis σmax 270 MPa for NbTi and 240 MPa for Nb3Sn Induction shading Cryocourse September

maximum Aluminum, stainless steel or fiber glass mandrel Make it possible to define the general shape of the coil to reach the

35 Impregnated NbTi superconducting solenoid Allows to choose the most appropriate superconducting wire Allows to design mandrel with material having the appropriate mechanical properties (define thickness) bigger wire in the center of the coil where the field is maximum Aluminum, stainless steel or fiber glass mandrel Make it possible to define the general shape of the coil to reach the specification (field homogeneity, maximum field, zero field zone etc.) Cryocourse September 2011 σθ JBzr with J : current density R: radius; Bz field 35

36 Impregnated NbTi superconducting solenoid Mechanical part design, machining Under 3D CAD Catia Cryocourse September

Impregnated NbTi superconducting solenoid Mandrel and flange insulation The goal is to do

of kapton or mylar thin sheet around the mandrel Use of thin fiber glass plate (0.

needed passage of wire before beginning the winding Envisage all place and screwed hole

37 Impregnated NbTi superconducting solenoid Mandrel and flange insulation The goal is to do electrical insulation between metallic mandrel, flange and the superconducting wire Use of kapton or mylar thin sheet around the mandrel Use of thin fiber glass plate (0.5-1 mm) against extremity flange Think about all wire entrances and outputs Envisage all needed passage of wire before beginning the winding Envisage all place and screwed hole you need to place contact, switch, resistance of protection, wire storage Cryocourse September

Impregnated NbTi superconducting solenoid Winding and impregnation The winding

performance Field shape Homogeneity Superconducting wire stability Control the

enough tension : low wire density, risk of wire movement during operation - due

38 Impregnated NbTi superconducting solenoid Winding and impregnation The winding The goal is to make the most constant and homogeneous winding Determine the coil performance Field shape Homogeneity Superconducting wire stability Control the wire tension - high tension : damage superconducting wire performance - not enough tension : low wire density, risk of wire movement during operation - due to high quantity of resin for impregnation, crack can easily appears Cryocourse September

39 The impregnation Impregnated NbTi superconducting solenoid To obtain a good insulation and a monolithic structure which cannot allow any movement of the conductor inside the coil Two techniques Vacuum Impregnation using epoxy resin Direct impregnation layer per layer with a brush Cryocourse September

40 Impregnated NbTi superconducting solenoid Vacuum Impregnation using epoxy resin Need of low viscosity resin (lower than 200 Mpa.S) the resin flow through the different layers of the coil and through the glass fiber cloth There is always the risk of bubble formation Creates regions of reduced thermal conductivity Risk of wire movement a risk of quench protect coil zones where we don't want resins after the process Well cleaning the impregnation device after use We must adapt the impregnation device for each different coil geometry expensive Essentials for Nb 3 Sn coil (because of heat treatment C)) Cryocourse September

Impregnated NbTi superconducting solenoid Direct impregnation layer per layer with a brush Difficult to obtain homogeneous impregnation Avoid to deposit high resins thickness ; risk of cracks

41 Impregnated NbTi superconducting solenoid Direct impregnation layer per layer with a brush Difficult to obtain homogeneous impregnation Avoid to deposit high resins thickness ; risk of cracks More easier to control bubble formation More easier to implement Need handmade work Good solution for laboratory coil which are all different the ones from the others Cryocourse September

42 Quench protection to allow the magnet to quench safely The quench Impregnated NbTi superconducting solenoid conversion of the magnetic energy into heat inside the volume of the winding which has transited into the resistive state Protection techniques consist in : decreasing the current as fast as possible without the appearance of over-voltages across or through the windings (max V ) because of thin insulated layers and low helium voltage breakdown (300 V/mm at 300 K) Cryocourse September

43 Quench protection Impregnated NbTi superconducting solenoid Discharge the energy into an external resistor A significant part of energy is also dissipated inside the winding due to the Quench back effect Quench back effect current decay magnetic flux variation eddy current in the coil mechanical structure losses heat cause the quench of others coil parts still in the superconducting state The result is a very fast growth of the coil resistance Cryocourse September

44 Quench detector Impregnated NbTi superconducting solenoid The device must detect a resistive voltage due to the quench which appear across the coil One difficulty is that the inductive voltage is one or two order of magnitude greater Electronic device allows to subtracting inductive voltage from the total magnet voltage The stored energy is extracted outside the coil into an external resistor by opening of breaker triggered by the quench detector Cryocourse September

Impregnated NbTi superconducting solenoid For impregnated NbTi laboratory magnet Not necessary to have quench detector Relatively low energy (hundred of kilojoules) High current densities (hundred of

45 Impregnated NbTi superconducting solenoid For impregnated NbTi laboratory magnet Not necessary to have quench detector Relatively low energy (hundred of kilojoules) High current densities (hundred of amperes per square millimeters) internal resistance increase rapidly The power source has its proper protective system against overvoltage If quench the power source is switched of The current is discharged through the free-wheel diode of the source A protective resistor must be mounted in parallel between the current lead terminals to prevent destructive damage in case of electrical line rupture Cryocourse September

46 Impregnated NbTi superconducting solenoid Superconducting persistent switch Used in order to increase magnet stability over long periods of time (L/R time constant is extremely long magnet can be operated for days or even months at a nearly constant field ) to reduce the rate of helium boil-off no need of continually supplying current to the magnet. Description persistent switch is comprised of a short section of superconducting wire connected across the input terminals of a magnet Special superconducting wires with resistive CuNi matrix (multifilament NbTi wires) an heater used to drive the wire into the resistive, normal state. Cryocourse September

47 Impregnated NbTi superconducting solenoid Superconducting persistent switch Operation description T room Power supply I the heater is turned on (wire is resistive) a voltage can be established across the terminals of the magnet magnet can be energized. the heater is turned off ( wire becomes superconducting changes in the magnet current cannot be made. R protection Cryocourse September T cryo In this persistent mode of operation, the external power supply can be turned off the heat input to the helium bath is reduced the current will continue to circulate through the magnet and the persistent switch without losses. Switch Heater Coil

Impregnated NbTi superconducting solenoid Superconducting persistent switch For a typical switch, the electrical heater in the persistent switch requires 300mW to drive the superconductor into the

48 Impregnated NbTi superconducting solenoid Superconducting persistent switch For a typical switch, the electrical heater in the persistent switch requires 300mW to drive the superconductor into the resistive state. The superconductive wire typically has 3 to 4 Ohms of resistance in the normal state special care must be taken in making the joints between the switch and the magnet leads. (To avoid current dissipation and then magnetic field decreasing). Magneto forming contact resistive CuNi matrix multifilament NbTi wires Heater Superconducting switch Cryocourse September

Impregnated NbTi superconducting solenoid Magneto forming contact (Magnetic Pulse Forming and welding ) The goal is to do an electrical contact with very

In the nearby sheet of metal an opposing magnetic field is induced.

49 Impregnated NbTi superconducting solenoid Magneto forming contact (Magnetic Pulse Forming and welding ) The goal is to do an electrical contact with very low resistivity A large current is discharged through a coil. The discharging current creates a magnetic field. In the nearby sheet of metal an opposing magnetic field is induced. The result is that the two magnetic fields oppose and a force moves the sheet away from the coil. Over a period of time the part is deformed, often to the shape of a mandrel The method generates pressures up to 50 Kpsi creating velocities up to 900 fps Cryocourse September

50 Impregnated NbTi superconducting solenoid Magneto forming contact Copper coil Copper mandrel NbTi wire without copper matrix (NbTi filaments) Copper tube Just before the current discharge Just after the current discharge Cryocourse September

51 Impregnated NbTi superconducting solenoid Magneto forming contact Copper tube R Ω Cross-sectional view Copper mandrel NbTi filament Cryocourse September

52 Impregnated NbTi superconducting solenoid Magneto forming contact Advantages, easy to control Process is repeatable no tool wear very strong joints Permanently joins dissimilar metals Heat-free welding process (No heat inherent in the process to degrade materials) Process is instantaneous Very low electrical contact resistance for us Cryocourse September

53 Impregnated NbTi superconducting solenoid Total system description T room Power supply T cryo Switch Electrical contact R protection Coil Cryocourse September

54 Impregnated NbTi superconducting solenoid Cryogenic tests, training, field card The last step consist in Coil training Coil characterization The means we need Liquid He cryostat with high section current leads High capacity helium recovery line to minimize pressure inside the cryostat during a quench Superconducting coil power supply (which automatically shut down in case of quench) Hall sensor to do the field card of the coil Helium level sensors to control helium consumption inside cryostat (particularly in case of quench) Current source to drive the superconducting switch Cryocourse September

filling Thermal shields Hole for field measure

measurement Helium recovery line Coil fiber glass

55 Impregnated NbTi superconducting solenoid Equipments for cryogenic tests Current leads Helium filling Thermal shields Hole for field measure (Stuffing box) Level, temperature, voltage measurement Helium recovery line Coil fiber glass support plate Superconducting coil power supply Cryocourse September

56 Training Impregnated NbTi superconducting solenoid When we immerse a superconducting coil device in a cryogenic liquid Due to thermal expansion for both superconducting wire and mandrel contraction appears in the axial and in the radial direction, (thick arrows A and B, C and D) In theory, no looseness can occur in the combination of these elements Cryocourse September

57 BUT During the first cooling cracks appears in the epoxy impregnation energy is released which cause disturbance that can affect the conductor stability. Consequence Impregnated NbTi superconducting solenoid We must applied to a new superconducting coil several thermal and electrical cycles before to obtain the optimal performances It is the training of the coil Cryocourse September

58 Training : Impregnated NbTi superconducting solenoid Electrical current pass through the superconducting coil electromagnetic force appears superconducting coil exhibits an increased degree of contraction in the axial direction, but expansion in the radial direction (thick arrows C' and E) looseness occurs in the combination of mandrel and superconducting coil to reduce this problem fiber glass binding band (to shrink) around the last layer of superconducting tape cracks in epoxy impregnation some wire can slightly "move" generation of frictional heat which can causes a quench. fiber glass binding band Cryocourse September

59 Training By doing several quench, the superconducting wire of the coil move toward the most stable position the consequence is a direct improvement in the coil characteristic Impregnated NbTi superconducting solenoid Thermal and electromechanical cycles Cryocourse September

60 cryogenic tests Impregnated NbTi superconducting solenoid Superconducting switch tests To validate persistent mode To see if there is no defective magneto forming contact Field card In case of defective contact, we observe a decrease of magnetic field (L/R) To compared with user's specifications To validate the coil Cryocourse September

2K Stainless steel and aluminum mandrel Zone of field homogeneity in

61 Others type of superconducting coils NbTi coils Two axis system (X, Y) High field main solenoid + helmoltz coil 8.6 T (Z) ; 1.5T (X) at 4.2K Stainless steel and aluminum mandrel Zone of field homogeneity in the center of the coil by playing with winding shape Cryocourse September

62 Others type of superconducting coils NbTi coils Two axis system (X, Y) High field main solenoid + helmoltz coil 5T (Z) ; 0.5T (X) Without metallic part only use of fiberglass (G11) 3T (Z) ; 2T (X) Cryocourse September

Others type of superconducting coils HTC superconducting coils With bismuth tape Very brittle Difficult to do helical coil : brittleness of the superconducting phase very difficult to keep the same

63 Others type of superconducting coils HTC superconducting coils With bismuth tape Very brittle Difficult to do helical coil : brittleness of the superconducting phase very difficult to keep the same length of tape between each layers (the tape "don't like" to be bend in the transverse direction) Solution Wind and react techniques : heat treatment to obtain the superconducting phase after windings To do pancake coil With superconducting strap to do electrical connection between each pancake coils Cryocourse September

HTC BISCO 32 double Pancake coils Operate at 20 K in vacuum Cooling

field 1.5 T operating current 210 A (tape section 0.33*4.

3% on 16 cm along the central axis of the coil 35 turns of

64 HTC BISCO 32 double Pancake coils Operate at 20 K in vacuum Cooling by conduction along the copper mandrel thanks to cryocooler Operating field 1.5 T operating current 210 A (tape section 0.33*4.5 mm² ) homogeneity of 0.3% on 16 cm along the central axis of the coil 35 turns of superconducting tape mylar insulation Copper mandrel one double pancake coils 11 cm Two double pancake coils Cryocourse September

65 Others type of superconducting coils 40 cm Thermal copper plates Tests with gifford macmahon cryocooler 32 double pancake coils Cryocourse September

66 Others type of superconducting coils SMES 26 pancake coils For electrical energy storage (800kJ) Operate in vacuum at 20 K Cooled with two cryocooler Superconducting tape "glued" on thermal copper plate Cryocourse September

67 References Handbook of cryogenic engineering J.G. Weisend II Cryogenic engineering Thomas M. Flynn Superconducting magnets Martin. N. Wilson Cryocourse September

2.3 PF System. WU Weiyue PF5 PF PF1

2.3 PF System. WU Weiyue PF5 PF PF1 2.3 PF System WU Weiyue 2.3.1 Introduction The poloidal field (PF) system consists of fourteen superconducting coils, including 6 pieces of central selenoid coils, 4 pieces of divertor coils and 4 pieces